1. Field of the Invention
The present invention relates to a developing device which is used in an image forming apparatus such as a copying machine, a printer, or the like, and develops an electrostatic image on an image carrier.
2. Related Background Art
In an image forming apparatus such as a copying machine, an image display apparatus, an image recording apparatus, a printer, a facsimile apparatus, or the like, a latent image formed on an image carrier, which comprises, e.g., an electrophotographic photosensitive body, an electrostatic recording dielectric member, or the like, is developed by a developing device and visualized as a toner image.
As an example of developing devices, various dry, one-component developing devices have been proposed and put into practical use. However, it is very difficult for these developing devices to form a toner thin layer of a one-component developing agent on a developing agent carrier. Since such a thin toner layer is currently required to improve sharpness, resolution, and the like of images, further development associated with a method of forming a thin toner layer and its associated apparatus is indispensable. Various proposals have been made to satisfy the above-mentioned requirements.
For example, a magnetic one-component toner has been used, a developing sleeve whose surface is subjected to blasting using regular particles to form a smooth surface having a relatively smooth three-dimensional pattern, and a magnetic blade which is arranged with a predetermined gap from the sleeve, all have used to form an appropriately triboelectrically charged thin toner layer on the sleeve.
In order to achieve higher image quality and a quick start of a copying operation, if a toner having a small particle size and a low melting point is used, a then blocking phenomenon of the toner easily may occur in the vicinity of the magnetic blade since the toner has a higher degree of agglomeration than that of a conventional toner, thereby resulting in considerably nonuniform or fogged images in a high-humidity environment. Also, in a low-humidity environment, the toner locally agglomerates and becomes attached (a blotch phenomenon) onto the sleeve due to a charge-up phenomenon in addition to the blocking phenomenon, and often appears on an image.
As a countermeasure against the above-mentioned phenomena, as described in U.S. Pat. No. 4,386,577, an elastic blade consisting of a rubber, resin, or metal material is kept in contact with the developing sleeve at a low pressure, and regulates thickness of a toner layer while removing the toner cohered on the sleeve by the contact portion, thus realizing formation of a uniform thin toner layer. In addition, since the upper and lower toner layers on the developing sleeve can be sufficiently triboelectrically charged by the blade, a high-quality image free from nonuniformity, fogging, and the like, can be produced.
However, upon replenishment of toner after continuous copying operations for tens of thousands of copies, a toner agglomerate locally clogs the contact portion between the elastic blade and the sleeve, thus causing coating nonuniformity of coating the of toner onto the sleeve.
In addition, the toner fuses on the contact surface of the elastic blade, and triboelectric charging performance of the toner is lowered.
In order to solve these problems, a roller which consists of open-cell polyurethane foam or and a fur brush structure and is rotated while contacting the sleeve at a position upstream in the sleeve rotational direction in the vicinity of the developing sleeve and the magnetic blade, so that toner adhered to the sleeve is swung by the contact portion and easily removed from the sleeve. At the same time, the toner is triboelectrically charged. In addition, the blocking toner in the vicinity of the blade imparts flowability upon rotation of the roller and can therefore maintain high fluidity. As a result, in the above-mentioned elastic blade system, a high-quality image free from blocking, nonuniformity, fogging, and the like can be produced.
Since an open-cell sponge roller suffers from a problem of a decrease in toner coating/removal performance of the roller or disintegration of the sponge roller (or a fur brush) caused by the roller which is hardened by the toner clogging inside the open-cell sponge roller, a developing device described in U.S. patent application No. 280,796 has been proposed. When a toner supply roller consists of closed-cell foam rubber free from internal toner clogging, the toner coating/removal performance with respect to the sleeve, and triboelectric charging performance of the toner can be satisfactorily maintained after copying operations of tens of thousands of copies even though the actual contact area increases as compared to an open-cell roller if the entrance amount of the toner to the sleeve remains the same.
On the other hand, when a non-magnetic toner is used, since a magnetic force cannot be used like in a magnetic toner, an arrangement described in, e.g., Japanese Laid-Open Patent Application No. 58-116559 has been proposed. More specifically, in this arrangement, the thickness of a toner layer on the sleeve is regulated by keeping the elastic blade in contact with the sleeve. A supply roller with a fur brush structure as a developing agent supply/peeling means is kept in rotatable contact with a portion on the upstream side in the sleeve rotational direction of the contact portion. With this arrangement as well, when the supply roller consists of a closed-cell foam roller, high-quality images can be stably provided even after continuous copying operations over a long period of time, as in the above-mentioned magnetic toner.
However, with a trend toward a higher resolution of a latent image in an image forming apparatus, reliance on the particle size of the toner as a developing agent is decreasing as a way to faithfully reproduce a high-resolution latent image, and the following problems arise.
More specifically, when the toner has a smaller particle size (becomes finer particles), the charge amount per unit area increases, and the degree of agglomeration increases at the same time. When toner in this state is supplied to a developing device to perform a copying operation, the toner clogs in the vicinity of the developing sleeve (blocking phenomenon), or coheres to the surface of the sleeve (blotch phenomenon).
In order to avoid these phenomena, a fine silica powder is externally added as a fluidity holding member in a quantity more than that added to a conventional toner so as to prevent an increase in degree of agglomeration.
However, when long-term copying operations are performed using a magnetic toner to which a large amount of the above-mentioned fine powder is externally added in the above-mentioned conventional system including the magnetic blade and the closed-cell supply roller, a decrease in density and generation of fogging gradually occur.
When the developing device was examined after an image output operation, it was found that the base layer portion of the surface of the closed-cell toner supply roller was cloudy. As a result of quantitative analysis of the composition of the cloudy portion, the cloudy portion included the silica as an external additive and the toner, and the weight % of the silica was abnormally larger than that in the initial state.
This phenomenon is caused by a difference between the charging systems of the silica in the toner and the closed-cell roller.
Normally, when a positive toner is used, positive silica is externally added; when a negative toner is used, negative silica is externally added, thereby preventing a decrease in charge amount of the toner due to addition of the external additive. Therefore, the toner and silica attract each other by a force such as an intermolecular force other than an electrostatic force. The closed-cell roller normally has an electrification polarity opposite to that of the toner so as to triboelectrically charge the toner using the contact portion between itself and the sleeve. In other words, the polarity of the silica is opposite to that of the closed-cell roller. When a copying operation is continued in this arrangement, since the amount of externally added silica is larger than that in the conventional toner, silica which cannot become attached to the toner surface with a strong force gradually coheres on the surface of the closed-cell roller electrostatically. As a result, the surface of the closed-cell roller is silica-coated, and the physical properties and shape of the surface change, resulting in a decrease in coating/removing performance of the supply roller. For this reason, since the charging characteristics and the like of the toner on the sleeve change from those in an initial state, a decrease in density, fogging, and the like occur. Such a phenomenon similarly occurs when a non-magnetic toner is used.
On the outer surface of the developing sleeve, as one constituent element of the developing device, a three-dimensional roughened surface is formed by, e.g., sand blasting for the purpose of improving toner carriability. In this case, since the closed-cell supply roller contacts the sleeve, a three-dimensional pattern finer than that in a system to which no roller contacts the sleeve can provide sufficient performance due to the coating effect.
When the present inventors conducted durability tests by outputting one million images using fine toner particles having a toner particle size of 6 .mu.m or less, the following problems were additionally posed.
(1) When a SUS (stainless steel) developing sleeve was used, its surface was subjected to sand blasting using regular beads to obtain a surface roughness Rz=3.0 .mu.m, and an image output operation was performed in a low-humidity environment where the initial solid black reflection density was 1.5, the absolute value of the charge amount of the toner on the sleeve was 20 .mu.c/g, and the weight per unit area of the toner on the sleeve (to be referred to as M/S hereinafter) was 1.4 mg/cm.sup.2. Thereafter, when several thousand images were output, the solid density became 1.2, the toner charge amount became 8 .mu.c/g, the M/S became 1.7 mg/cm.sup.2, and the amount of toner attached to a non-image portion (to be referred to as fogging hereinafter) increased. However, when the image output operations were further continued, the density and fogging were improved. When five hundred thousand images were output, the density became 1.4 to 1.5, the charge amount became 20 .mu.c/g, the M/S became 1.0 mg/cm.sup.2, and the one millionth image was output in this state. In the latter half of the image output durability tests, several stripe-shaped portions on which no toner was attached were generated in the sleeve rotational direction (this phenomenon will be referred to as "white stripe" hereinafter).
(2) An Al sleeve was used, its surface was subjected to the same surface roughening treatment as in (1), and image output operations were similarly performed. As a result, when several thousand images were output, a decrease in density, fogging, and white stripes were similarly observed. Thereafter, in this case as well, the density and fogging were improved, and stable image quality was maintained after the ten thousandth image.
When the cause of the above-mentioned phenomenon was examined, it was found that a decrease in density and an increase in fogging after image output operations of several thousand images in the case of the SUS sleeve were caused by a decrease in triboelectric charging performance from the sleeve to the toner and an increase in M/S due to finer particles in the fine toner particles, which cohered on small recess portions of the three-dimensional roughened surface of the sleeve.
The reason why the density and fogging were improved later was as follows. That is, as the number of times of sliding contact between the closed-cell roller and the sleeve increased, the three-dimensional pattern on the surface of the sleeve gradually was wore and smoothed, and the toner cohered on the surface was removed together with the worn powder of the sleeve. As a result, the toner charge amount increased, and carriability was stabilized.
Since the Al sleeve was softer than the SUS sleeve, its surface wore earlier than that of the SUS sleeve, and hence, the density and the like were improved earlier than the SUS sleeve.
Note that white stripes in the circumferential direction of the developing sleeve were generated since coarse particles of the worn powder of the sleeve were deposited between the magnetic blade and the sleeve.